The present invention relates to a single crystal wafer of rare earth gallium garnet bearing an epitaxially grown magnetic layer thereon. More particularly, the invention relates to a so-called epitaxial single crystal wafer of rare earth gallium garnet suitable as an element of a MSW (magnetostatic wave) device such as MSW filters, oscillators, isolators, circulators and the like.
It is an established technology in the field of microwave engineering to use a single crystal of YIG (yttrium iron garnet) grown by the flux method as a magnetic material for microwave devices. Such a microwave device manufactured by using a YIG single crystal grown by the flux method, however, is disadvantageous due to expensiveness of the single crystal as a consequence of the low productivity in the process of single crystal growing. Accordingly, it is proposed to use a so-called epitaxial YIG single crystal grown on a suitable substrate single crystal wafer of, for example, GGG (gadolinium gallium garnet) by the method of liquid-phase epitaxy (LPE) utilizing the stockpile of technology obtained in the manufacture of bubble memory devices.
Various kinds of rare earth gallium garnets, generally referred to as the RGG hereinbelow, are proposed for a single crystal material used as a substrate wafer on which the epitaxial layer of YIG is grown in the above mentioned LPE method. One of the important problems involved in these RGG single crystals as a substrate for YIG epitaxy is that no good matching can be obtained in the lattice constants between the substrate RGG and the epitaxial YIG. Namely, while single crystals of YIG have a lattice constant of 1.2376 nm, all of the hitherto proposed RGG single crystals have a significantly different lattice constant including 1.2383 nm in GGG, 1.245 nm or 1.2946 nm in GGG partly substituted with calcium, magnesium or zirconium, 1.2438 nm in SGG (samarium gallium garnet), 1.2508 nm in NGG (neodymium gallium garnet) and so on. Mismatching in the lattice constants between the substrate and the epitaxial layer naturally causes a large stress in the epitaxial layer which may eventually lead to cracking of the epitaxial layer. It is eagerly desired therefore to develop an inexpensive and reliable epitaxial single crystal wafer of RGG suitable as a microwave device or, in particular, MSW device.